The suspension polymerization of vinyl chloride is generally carried out at temperatures below 70.degree. C. using organic soluble initiators. Although lauroyl peroxide was earlier the most widely used catalyst, in recent years other low temperature catalysts including azobisisobutyronitrile, diisopropyl peroxydicarbonate, t-butyl peroxypivalate and mixtures thereof, have been adopted. These and other catalysts are described in Pennwalt Corporation, Lucidol Division, Technical Bulletin 30.90, "Free Radical Initiators for the Suspension Polymerization of Vinyl Chloride" (1977).
The choice of initiator is dictated by its half-life and by its influence on the polymerization process and the properties of the poly(vinyl chloride) produced thereby.
The polymerization of vinyl chloride is characterized by a short induction period, followed by a gradually increasing rate of polymerization. During the earlier stages of the polymerization, the reaction rate is lower than the maximum so that the capacity of the reactor is not fully utilized. Peroxyesters reduce the induction period and, due to a more constant rate of polymerization, increase reactor productivity. Further, peroxyesters can generally be used at levels below that needed for peroxides and give much less chain branching during polymerization.
Although peroxyesters such as diisopropyl peroxydicarbonate and t-butyl peroxypivalate offer numerous advantages in vinyl chloride polymerization, their disadvantages include the necessity for low temperature shipping and storage and decreased efficiency at elevated temperatures.
The use of peroxyesters having higher decomposition temperatures is not feasible in present poly(vinyl chloride) production facilities due to the higher monomer pressures involved and the low molecular weight and poorer stability of the resultant resins. Nevertheless, the handling advantages of such peroxyesters makes their use extremely attractive.
The use of higher temperature catalysts at lower temperatures is a common practice in polymer technology. Thus, redox systems such as ammonium persulfate--sodium metabisulfite and hydrogen peroxide--ferrous sulfate are used in emulsion polymerization while benzoyl peroxide--dimethylaniline and methyl ethyl ketone peroxide--cobalt naphthenate are used in styrene--unsaturated polyester polymerization.
Reducing agents used in conjunction with monomer-soluble peroxyesters in the polymerization of vinyl chloride include potassium metabisulfite (N. Fischer and C. Lambling, French Patent No. 2,086,635 (1972), sodium bisulfite (H. Minato, K. Hashimoto, and T. Yasui, Japan. Patent No. 68 20,300 (1968), sodium bisulfite-cupric chloride (B. K. Shen, U.S. Pat. No. 3,668,194 (1972), sodium dithionite-ferrous sulfate (H. Minato, Japan. Patent No. 70 04,994 (1970) and trialkyl boron (R. Kato and I. Soematsu, Japan Patent No. 5498('65) (1965); A. V. Ryabov, V. A. Dodonov, and Y. A. Ivanova, Tr. Khim. Khim. Tekknol., 1970, 238; Stockholms Superfosfat Fabriks A/B, Brit. Patent No. 961,254 (1964).
Reducing agents used in conjunction with monomer-soluble diacyl peroxides in the polymerization of vinyl chloride include ferrous sulfate-sodium hydroxide (A. M. Sharetskii, S. V. Svetozarskii, E. N. Zil'berman, and I. B. Kotlyar, Brit. Patent No. 1,164,250 (1969) and U.S. Pat. No. 3,594,359 (1971), ferrous caproate (J. Ulbricht and N. V. Thanh, Plaste Kaut., 21, 186 (1974); J. Ulbricht and G. Mueller, Plaste Kaut., 21, 410 (1974) and trialkyl boron (A. V. Ryabov, V. A. Dodonov, and Y. A. Ivanova, Tr. Khim. Khim. Tekknol., 1970, 238).
Ascorbic acid has been used as the sole reducing agent or in combination with cupric, ferrous or ferric salts, in the polymerization of vinyl chloride in the presence of water-soluble catalysts including hydrogen peroxide (H. I. Roll, J. Wergau and W. Dockhorn, Ger. Offen. Patent No. 2,208,442 (1973); J. A. Cornell, U.S. Pat. No. 3,534,010 (1970); K. Okamura, K. Suzuki, Y. Nojima and H. Tanaka, Japan. Patent No. 18,954('64) (1964); H. Watanabe, S. Yamanaka and Y. Amagi, Japan. Patent No. 16,591('60) (1960), potassium persulfate (K. H. Prell, E. Plaschil and H. Germanus, East Ger. Patent No. 75,395 (1970), cumene hydroperoxide (R. J. S. Matthews, Brit. Patent No. 931,628 (1963), acetyl cyclohexanesulfonyl peroxide (Dynamit Nobel A. G., Netherlands Appl. No. 6,408,790 (1965), and a mixture of hydrogen peroxide and acetyl cyclohexanesulfonyl peroxide (R. Buning, K. H. Diessel and G. Bier, Brit. Patent No. 1,180,363 (1970).
Ascorbic acid has been disclosed as a complexing agent in the polymerization of vinyl chloride in the presence of a diacyl peroxide and various water-soluble metal salts (N. Fischer, J. Boissel, T. Kemp and H. Eyer, U.S. Pat. No. 4,091,197 (1978).
6-O-Palmitoyl-L-ascorbic acid has been used as reducing agent in the polymerization of vinyl chloride in the presence of hydrogen peroxide (K. Kamio, T. Tadasa and K. Nakanishi, Japan. Patent No. 71 07,261 (1971) and methyl ethyl ketone peroxide (K. Kamio, T. Tadasa and K. Nakanishi, Japan. Patent No. 70 25,513 (1970).
The water-soluble peroxygen compounds and reducing agents are more suitable for emulsion than for bulk or suspension polymerization, the trialkyl borons react with oxygen and require special handling and the presence of the various metal compounds such as copper and iron salts is detrimental to the properties of poly(vinyl chloride).